Chalcogenide materials are materials that contain a chalcogen element (O, S, Se, Te) and typically one or more additional elements that serve to modify electronic or structural properties. The II-VI semiconductors (e.g. CdS, ZnTe etc.) are a well-known class of chalcogenide materials. These materials have been widely investigated for their wide bandgap properties and their potential for providing short wavelength light emission for LED and laser applications.
Another important class of chalcogenide materials includes the expansive series of chalcogenide materials, initially developed by S. R. Ovshinsky, that are currently being used in optical and electrical memory and switching applications. These chalcogenide materials may be referred to herein as Ovonic chalcogenide materials. Among the Ovonic chalcogenide materials are chalcogenide phase change materials that are currently widely in use in optical recording technologies. The active materials in CD and DVD applications are chalcogenide materials that have a crystalline state and an amorphous state whose relative proportions can be reversibly and reproducibly varied through the application of optical energy. These materials can be used to store information by defining a series of two or more distinct structural states, each of which is defined by a characteristic proportion of crystalline and amorphous phase domains within a given volume, and associating a distinct information value to each structural state. Storage of an information value occurs by applying optical energy to the phase change material in an amount necessary to convert the material to the structural state associated with the information value.
The optical phase change chalcogenide materials are reversibly transformable between different structural states through the judicious application of energy. The proportion of amorphous phase can be increased by applying energy sufficient to create a local temperature in the phase change material that exceeds the melting temperature and removing the energy at a rate sufficient to prevent crystallization upon cooling. The proportion of crystalline phase can be increased by applying energy sufficient to create a local temperature in the phase change material that exceeds the crystallization temperature so that a controlled transformation of amorphous phase material to crystalline phase material is induced. Reading of the information content of the phase change material occurs through the detection of a physical characteristic of the structural state of the material. In optical recording, for example, reflectivity is a widely used as a parameter for detecting the structural state. The reflectivity difference between the crystalline and amorphous states provides sufficient contrast to permit clear resolution of structural states that differ with respect to the relative proportions of crystalline and amorphous phase volume fractions.
Two other important types of Ovonic chalcogenide materials are the electrical switching and electrical memory materials. The Ovonic electrical switching chalcogenide materials are switchable between a resistive state and a conductive state upon application of a threshold voltage. In the resistive state, the materials inhibit the flow of electrical current and upon application of the threshold voltage, the material switches nearly instantaneously to its conductive state to permit the flow of current. In the Ovonic electrical memory materials, application of electrical energy (typically in the form of current pulses) induces changes in the structural state of the chalcogenide material. The relative volume fraction of crystalline and amorphous phase domains can be continuously varied through judicious control of the duration and magnitude of a series of one or more applied current pulses. Each structural state has a unique resistance and each resistance value can be associated with a distinct information value. By applying an appropriate current pulse, the electrical chalcogenide memory material can be programmed into the resistance state that corresponds to a particular information value to write that value to the material. The electrical memory material can be transformed among its different resistance states to provide erasing and rewriting capabilities. Both the electrical and optical chalcogenide memory materials can be incorporated into arrays to provide advanced, high density memory capability.
As the appreciation of the range of applications of available from chalcogenide materials grows, greater attention is being placed on further understanding their properties and on developing new chalcogenide materials that exhibit a wider range of properties. The development of new materials requires the synthesis or deposition of either new compositions or existing compositions having unique microstructures. The primary preparation methods for the optical and electrical chalcogenide materials are sputtering and physical vapor deposition. Although these techniques have provided for a number very interesting and useful materials, it is expected that the development of new synthetic or preparation methods will expand the range of compositions and properties of chalcogenide materials and will further the objective of expanding the applications of chalcogenide materials.
Chemical vapor deposition, hereinafter referred to as CVD, is a widely used technique for the synthesis of materials. In the CVD process, precursors of the constituent elements of a material are reacted to produce a thin film on a substrate. The reaction of the CVD precursors occurs either homogeneously in the gas phase or heterogeneously at the solid-gas interface of the substrate surface. Precursors for many elements are available and a variety of thin film compositions can be synthesized using CVD.
In CVD processing, precursors are introduced into the reactor in gas phase form. Precursors that are in the gas phase at room conditions are directly introduced into the reactor, typically in diluted form via a carrier gas. Liquid and solid phase precursors are vaporized or sublimed and then introduced into the reactor, also typically in diluted form in the presence of a carrier gas. Upon introduction into the reactor, precursors containing the chemical constituents of the desired material are decomposed (thermally, photochemically, or in a plasma) to provide intermediate species of the constituents that subsequently react to form a thin film of desired composition. The rate of deposition, stoichiometry, composition and morphology of the film can be varied through appropriate control over process parameters such as reaction temperature; substrate; selection of precursor; reactor pressure; and the rate of introduction of precursors into the reactor. CVD offers the advantages of providing high purity thin films at relatively low temperatures.
Although CVD, has been widely used for II-VI materials that contain chalcogenide elements and simple binary chalcogenides such as Sb2Te3, its use for the Ovonic family of optical and electrical chalcogenide materials has been virtually non-existent due to the anticipated difficulties associated with producing the multiple element (ternary and higher) compositions typically associated with the most effective optical and electrical switching and memory chalcogenide materials. CVD synthesis of the optical and electrical switching and memory chalcogenides is an outstanding challenge that remains to be addressed. Successful development of the CVD synthesis of these materials is expected to provide a wider range of compositions with more diverse switching, memory and phase change characteristics and accordingly will provide new materials that can fulfill the ever-increasing expectations for chalcogenide materials.